A magnetic resonance imaging apparatus creates images of a subject's internal body by use of magnetic resonance phenomenon. Specifically, the magnetic resonance imaging apparatus applies a high frequency magnetic field to the subject positioned in a magnetostatic field, and detects a nuclear magnetic resonance (NMR) signal output from the subject in response to the application. Then, the magnetic resonance imaging apparatus reconstructs images from the detected NMR signal.
Conventionally, when an image of an abdomen or the like is to be taken by use of the magnetic resonance imaging apparatus, imaging is conducted by having the subject hold its breath or by synchronizing the imaging with breathing. The breath-holding imaging method is performed while the subject patient is holding its breath for about 10 seconds. The breath-synchronizing imaging method is performed by putting gear such as bellows on the patient and monitoring the patient's respiration state so that imaging can be synchronized with breathing.
Both the breath-holding imaging and the respiration-synchronizing imaging are targeted at reduction of artifacts that tend to be created in an image due to respiration. In the breath-holding imaging, however, drawbacks have been pointed out that the spatial resolution is substantially limited by the length of breath-holding time and that the method is not suitable for a patient who cannot hold its breath. Furthermore, in the respiration-synchronizing imaging, drawbacks have been pointed out that it is difficult to obtain stability in the sensitivity of the gear and that the data collecting timing is limited to the stable respiratory phase and therefore the entire imaging time becomes longer.
Recently, a respiration-synchronizing imaging method with which the diaphragm in respiration is monitored by use of NMR signals has been introduced (see, for example, Klessen et al., “Magnetic Resonance Imaging of the Upper Abdomen Using a Free-breathing T2-weighted Turbo Spin Echo Sequence with Navigator Triggered Prospective Acquisition Correction”, Journal of Magnetic Resonance Imaging 21, 2005, pp. 576 to 582).
FIGS. 12A, 12B, and 13 are diagrams for explaining the conventional respiration-synchronizing imaging method adopting NMR signals. As indicated in FIG. 12A, with the conventional method, the magnetic resonance imaging apparatus takes a coronal image 81 of the abdomen by executing a navigator sequence (respiration monitoring sequence) such as a 2D gradient echo sequence, and obtains, as a profile, an NMR signal from a one-line equivalent region 82 including the diaphragm that is set in the coronal image 81. By repeatedly collecting the profile, the diaphragmatic motion can be intermittently detected (at intervals of 150 milliseconds in general).
Furthermore, with the above method, the magnetic resonance imaging apparatus usually detects a position 84 of the diaphragmatic motion, as a relative motion amount with respect to a reference profile 83, as illustrated in FIG. 12B. Then, the magnetic resonance imaging apparatus generates a trigger signal in a phase with a small amount of diaphragmatic motion, based on the detected amount of motion, and executes an imaging sequence in response to the trigger signal. In other words, the magnetic resonance imaging apparatus executes the imaging sequence during a respiration phase with a small amount of motion only.
In addition, the magnetic resonance imaging apparatus resumes a navigator sequence 92 several hundreds of milliseconds after executing an imaging sequence 91, as illustrated in FIG. 13. Here, a vacant time is provided between the imaging sequence 91 and the navigator sequence 92 because the signal excited in the imaging sequence 91 should be prevented from affecting, as saturation effects, the image taken in the navigator sequence 92.
With the conventional method, the magnetic resonance imaging apparatus alternately executes the navigator sequence and the imaging sequence. The above explanation focuses on the respiration-synchronizing imaging, but the magnetic resonance imaging apparatus executes the same sequences and calculation process in the breath-holding imaging also.
With the above method, however, there has been a problem of a long period of time required for imaging, as explained below.
For example, as described above, the magnetic resonance imaging apparatus according to the conventional method executes the imaging sequence only in the respiration phase with little motion. Thus, with the conventional method, the imaging sequence has to be repeated in every respiration phase with little motion until all the necessary data is collected, and therefore the entire period of time for imaging becomes very long. If the period of time for one data collecting operation is extended to reduce the imaging time, the influence of the motion would become large, and artifacts would increase in an image that is taken in the imaging sequence.
In addition, the navigator sequence that cannot be resumed immediately after the imaging sequence further extends the imaging time. If the navigator sequence is resumed immediately after the imaging sequence to reduce the imaging time, a low signal region would be created in an image taken in the navigator sequence, which makes it difficult to accurately calculate the position of the diaphragm.